1. Field of the Invention
This invention relates to a receiver or the like that uses multiple antennas to receive signals arriving over multiple propagation paths, and particularly to a technology that improves reception quality by enhancing the accuracy of received signal path detection.
2. Description of the Prior Art
A receiver for wireless reception of direct sequence-code division multiple access (DS-CDMA) signals or other such spread-spectrum communication system signals generally demodulates the desired incoming waves by correlating the received signal with a spreading code at a timing synchronized with the desired incoming waves contained in the received signal. Therefore, in the CDMA system, for instance, the first processing step is to detect the timing of the desired incoming waves in the received signal.
Detection of incoming wave timing is customarily called path detection. Moreover, a signal from a transmitter may arrive at the receiver via multiple paths, in which case multiple paths exist for that particular signal.
In recent years, consideration has been given to CDMA transceiver circuits that utilize adaptive array antennas (AAAs). Such a CDMA transceiver circuit uses a CDMA system to transmit and receive signals over the airwaves utilizing an adaptive array antenna.
At the time of signal reception, the multiple antennas of the adaptive array antenna are generally weighted to adjust the receive directivity to enable maximum directivity in the direction of the desired incoming wave and strongly depress received signal quality with respect to signals from other directions. At the time of signal transmission, the multiple antennas of the adaptive array antenna are weighted to adjust the transmit directivity to enable maximum directivity in the desired direction and strongly depress transmit signal quality with respect to other directions.
FIG. 5 shows an example of receive directivity patterns of an adaptive array antenna. The pattern designated (a) is an example in which the maximum directivity has been adjusted to 0 degrees for reception of an incoming wave from the 0-degree direction, and the pattern designated (b) is an example in which the maximum directivity has been adjusted to 45 degrees for reception of an incoming wave from the 45-degree direction. For reference, the drawing also shows the directivity component in the 180-direction that is the opposite direction from 0 degree, and the −45-degree direction that is the opposite direction from 45 degrees.
When an adaptive array antenna capable of achieving this kind of directivity is used to receive and process signals, the desired signal can be received while eliminating interference entering the antenna owing to waves arriving from directions different from the arrival direction of the desired signal. The adaptive array antenna has therefore drawn considerable attention as a technology for eliminating interference waves received by the antenna owing to waves arriving from directions different from the arrival direction of the desired signal. The adaptive array antenna has therefore drawn considerable attention as a technology for eliminating interference.
FIG. 6 shows an example configuration of a receiver including a path detection circuit used in a conventional base station equipped with an adaptive array antenna.
The receiver comprises N number (a plural number) of receive paths composed of N number of antennas G1–GN constituting an adaptive array antenna, N number of receiver units (RX) H1–HN each associated with one of the antennas G1–GN, and N number of user separators I1–IN each associated with one of the antennas G1–GN (and one of the receiver units H1–HN). The illustrated receiver also includes a user-segregated AAA signal processor and discriminator 41 common to the N number of receive paths.
The illustrated receiver is further equipped with a path detection circuit composed of an antenna 42, receiver unit 43, spreading code generator 44, correlator 45, delay profile analyzer 46 and path detector 47.
Each of the N number of antennas G1–GN receives wireless signals.
Each of the N number of receiver units H1–HN down-converts the input signals from the associated antenna G1–GN from a carrier frequency band signal to a baseband signal and outputs the down-converted signal to the associated user separator I1–IN.
Although in the illustrated example the receiver units H1–HN down-convert the input signals from the antennas G1–GN to baseband signals and then output the baseband signals to the user separators I1–IN, other configurations are also usable.
The user separators I1–IN include multiple correlators (commonly referred to as “fingers”) in a number equal to ({number of users (mobile stations)}×{number of paths per user}), which they use to separate the signals input from the associated receiver units H1–HN into signals of the individual users and individual paths. They then output the separated signals (user-separated signals) to the user-segregated AAA signal processor and discriminator 41.
FIG. 7 shows an example configuration of the correlators provided in the user separators I1–IN. The correlator is equipped with a user-segregated spreading code generator 51, complex multiplier 52, synthesizer (adder) 53, delay element 54 and switch 55.
An example of the operation of the illustrated correlator will be explained.
The user-segregated spreading code generator 51 generates a user-specific spreading code defined for each user and outputs the generated spreading code to the complex multiplier 52.
The complex multiplier 52 performs complex-multiplication on the spreading code from the user-segregated spreading code generator 51 and the signal from the associated receiver unit H1–HN every chip and outputs the result of the multiplication to the synthesizer 53.
The synthesizer 53 synthesizes (adds) the multiplication result for every chip input from the complex multiplier 52 and the output of the delay element 54 (explained later) and outputs the result of the synthesis.
The switch 55 is controlled by, for instance, a controller to open for one spreading code period of the user and to close when the period has elapsed. When the switch 55 is open, the synthesis result output by the synthesizer 53 is input to the delay element 54. When the switch 55 is closed, the synthesis result output by the synthesizer 53 is output through the switch 55 to the user-segregated AAA signal processor and discriminator 41.
The delay element 54 delays the synthesis result received from the synthesizer 53 by a prescribed time period and outputs it to the synthesizer 53. As a result, the multiplication results output by the complex multiplier 52 for a number of successive spreading code chips are cumulatively synthesized by the synthesizer 53 and the cumulative synthesis result is output through the switch 55 to the user-segregated AAA signal processor and discriminator 41. The cumulative synthesis results output through the switches 55 of the respective user separators I1–IN correspond to the result of correlating the signals output by the user separators I1–IN and the respective user spreading codes (despreading result) and thus correspond to signals separated by user and path.
The user-segregated AAA signal processor and discriminator 41 includes a user-segregated AAA signal processor and a discriminator.
The user-segregated AAA signal processor multiplies the user separated signals received from the user separators I1–IN and individual user receive weights (complex coefficients for weighting) and acquires the synthesized (summed) result of the multiplication results (in this example, N number of multiplication results based on the outputs from the antenna paths) as the adaptive array antenna receive result.
The discriminator discriminates the received data based on the synthesis result acquired by the user-segregated AAA signal processor and outputs the discriminated individual user data signals as demodulated signals (from the user-segregated AAA signal processor and discriminator 41).
The user-segregated AAA signal processor and discriminator 41 thus outputs the user data of each of M number (M being one or greater) of users.
The antenna 42 of the path detection circuit receives a wireless signal.
The receiver unit 43 of the path detection circuit down-converts the input signal from the antenna 42 from a carrier frequency band signal to a baseband signal and outputs the down-converted signal to the correlator 45. Although in the present example the receiver unit 43 down-converts the input signal from the antenna 42 to a baseband signal and the baseband signal is thereafter subjected to correlation processing in the correlator 45, other configurations are also usable.
The spreading code generator 44 generates user-specific spreading codes defined for the respective users and outputs the generated spreading codes to the correlator 45.
The correlator 45 correlates the signal received from the receiver unit 43 with the spreading code received from the spreading code generator 44 and outputs the correlation result to the delay profile analyzer 46.
A matched filter (MF) is generally used as the correlator 45. The matched filter is an infinite impulse response (FIR) filter. In the present example, its multiplication coefficient is the user-specific spreading code supplied by the spreading code generator 44.
FIG. 8 shows an example configuration of the correlator (MF) 45 and a spreading code generator 61 that corresponds to the spreading code generator 44 shown in FIG. 6. The correlator 45 comprises L number of series-connected shift registers J1–JL, L number of complex multipliers K1–KN each connected to the output of one of the shift registers J1–JL, and a synthesizer 62. L is equal to the number of chips contained in one spreading code.
The first shift register J1 receives the signal from the receiver unit 43 (e.g., an I signal or Q signal). It outputs the signal to the first complex multiplier K1 and, after a prescribed delay time, also outputs it to the second shift register J2.
Similarly, among the second to (L−1)th shift registers J2–JL−1, the ith (i=2–L−1) shift register receives the signal output by the preceding shift register Ji−1, outputs the signal to the ith complex multiplier Ki, and, after a prescribed delay time, also outputs it to the next shift register Ji+1.
The Lth shift register JL receives the signal output by the (L−1)th shift register JL−1 and outputs it to the Lth complex multiplier KL.
The spreading code generator 61 outputs L number of values (e.g., 1 and 0 values) corresponding to the L number of chips making up one spreading code to the complex multipliers K1–KL, using a bus line, for example.
The complex multipliers K1–KL perform complex multiplication on the signal values input from the shift registers J1–JL and the values input from the spreading code generator 61 and output the multiplication results to the synthesizer 62.
The synthesizer 62 synthesizes the L number of multiplication results received from the L number of complex multipliers K1–KL and outputs the synthesis result to the delay profile analyzer 46.
Operating in this manner, the correlator 45 successively correlates the received signals (their I or Q signals) from the antenna 42 with the spreading codes. When the spread spectrum signal contained in the received signal (its spreading code) and the spreading code output by the spreading code generator 44 (61) match in phase, a sharp autocorrelation peak peculiar to the spreading code appears in the output of the correlator 45.
Moreover, in the case where, for example, a delayed-wave path of the signal occurs in the propagation path in addition to the leading wave (direct wave) path of the signal, an autocorrelation peak of the delayed-wave appears in the output of the correlator 45 at a point shifted timewise from the leading wave autocorrelation peak by the delay time of the delayed-wave path.
FIG. 9 shows an example of the correlation result output by the correlator 45. Relative delay time is represented on the horizontal axis of the graph and the output level (receive level) of the correlation result on the vertical axis. The graph shows a case in which two propagation paths are present and the noise level Eb/N0=100 db.
The correlation result shown by the graph indicates the delay profile of the propagation path. Specifically, in this example, one delayed-wave has occurred relative to the leading wave in the propagation path and, as a result, two autocorrelation peaks have appeared.
A case in which noise and interference were reduced to the lowest possible level (Eb/N0=100 db) was taken as an example here in order to simplify the state of the propagation path. Actually, however, the autocorrelation peaks are buried in noise and interference. In the example under discussion, the buried state of the autocorrelation peaks is mitigated, as explained later, by having the delay profile analyzer 46 average the output of the correlator 45 over a long interval (long time period)
In other words, a time-averaged delay profile acquired by causing the delay profile analyzer 46 to average the correlation result received from the correlator 45 over time is output to the path detector 47.
FIG. 10 shows an example of cumulatively summed and averaged output of the correlator 45 (averaged data) and an example of a threshold Q explained below. The horizontal axis of the graph represents relative delay time and the vertical axis averaged data P and threshold Q.
As shown in FIG. 10, the path detector 47 sets an appropriate threshold Q with respect to the averaged delay profile received from the delay profile analyzer 46. Paths and noise are distinguished by defining averaged data P portions exceeding the threshold Q as autocorrelation peaks and defining averaged data portions equal to or lower than the threshold Q as noise portions. This enables detection of the path arrival times (leading wave and delayed-wave arrival times).
The so-detected path arrival times are, for example, imparted to the correlators (fingers) in the user separators I1–IN. The correlators can therefore generate spreading codes synchronously with the path arrival times and use them to carry out the correlation processing. The generation of the spreading codes with such timing establishes synchronization that constantly enables maximum value autocorrelation peaks of the desired incoming waves to be acquired in the correlation processing.
However, the detection circuit of a conventional CDMA receiver equipped with an adaptive array antenna, of which the CDMA receiver illustrated in FIG. 6 is one example, has a drawback in that the path detection accuracy in the path detector 47 is low because no adaptive array antenna processing is applied in the path detection.
This drawback will be explained.
In the CDMA system, for example, transmission power control is conducted that makes it possible to reduce the transmission power required to communicate with a receiver using an adaptive array antenna and capable of interference level reduction (compared with the case of not using an adaptive array antenna). In other words, the receiver can enhance the received signal quality by using the adaptive array antenna to receive and process the signal. However, the path detector 47 is not equipped with an adaptive array antenna. This not only prevents improvement of the path detection accuracy but also leads to degraded path detection accuracy when the transmission power is reduced.
Thus, the conventional receiver would be able utilize the adaptive array antenna to enable reduction of transmission power on the transmission side if the detection of path timing were as accurate as it might be. In fact, however, when the transmission power is reduced, the accuracy of path detection decreases to the point that the timing of autocorrelation peaks cannot be accurately detected. This makes it impossible to lower the transmission power.
To say that the transmission power cannot be reduced is the same as saying that the transmission power per user increases. In other words, in the CDMA system, which views signal power from users other than the desired user as interference power, the maximum number of mobile stations (users) that a base station can accommodate (the base station capacity) decreases. This is a serious problem.
Since the conventional CDMA system utilizing the adaptive array antenna thus requires high transmission power to prevent degradation of path detection accuracy, it therefore has the drawback of small system capacity. That is, the enhanced demodulation accuracy of a system utilizing the adaptive array antenna as a rule enables lowering of the transmission power on the transmission side, but reduction of the transmission power lowers the signal-to-noise ratio (SNR) of the conventional receiver described in the foregoing with respect to the received signal and makes it incapable of accurate path detection. Reduction of transmission power is therefore impossible.
A conventional technology related to path detection will be explained.
In a paper titled “Proposed CDMA path search method using an antenna synthesized delay profile” (Collected Papers for Presentation at 1999 Conference of Communications Society, The Institute of Electronics, Information and Communication Engineers B-5-39), Aoyama, Yoshida and Atokawa propose a path search method for path detection in a CDMA receiver that conducts RAKE receive. The method achieves high-speed, high-accuracy path detection by detecting path timing common to every antenna based on a synthesized delay profile that is the result of synthesizing the individual delay profiles of multiple diversity antennas. Unlike the present invention, however, the proposed method does not utilize the weights the individual antennas.
On the other hand, in a paper titled “Path search method suitable for a W-CDMA system using an adaptive array antenna” (Collected Papers for Presentation at 2000 Conference of The Institute of Electronics, Information and Communication Engineers B-5-53), Jitsukawa, Tsutsui and Tanaka propose a path search method that enhances path detection accuracy by generating a voltage profile for every antenna element making up the adaptive array antenna (i.e., in essence detecting the path for each antenna), estimating the phases of the multiple voltage profiles, and conducting the search using a delay profile obtained by in-phase synthesis of the multiple voltage profiles. (This paper will hereinafter be called “Reference A”). Unlike the present invention, however, the proposed method requires a large-scale physical (circuit) configuration because it has to separately calculate voltage profile for all of the multiple antennas and therefore needs multiple voltage profile calculating sections for this purpose.
The present invention was accomplished in light of the foregoing circumstances and has as an object to provide a receiver, CDMA receiver and CDMA base station that enhance received signal path detection accuracy when receiving a signal arriving via multiple paths using multiple antennas.
Another object the present invention is to provide a path detector and a path detection method that enhance received signal path detection accuracy.